Maintaining the correct temperature in your engine is vital to its long-term reliability.
Should the cooling system fail, your engine could overheat, causing catastrophic failure and an expensive repair bill.
The radiator cap is an integral part of your cooling system. It controls the pressure in your radiator and allows coolant to flow into the expansion bottle to maintain the correct cooling system pressure.
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What Problems Can a Loose Radiator Cap Cause?
A loose radiator cap will fail to maintain the pressure inside the radiator. As the engine heats up, coolant levels will rise as it expands, causing it to push against the radiator cap. The loose radiator cap will then allow the fluid to overflow, resulting in the engine overheating.
The engine overheating will cause the oil to become thin and lose its ability to lubricate the metal surfaces. This will in turn generate huge amounts of heat. The metal will expand, bearings will seize and the pistons will jam in the bores.
This disintegration of the engine can happen very quickly and will be accompanied by lots of noise and smoke from the exhaust. The intense heat may even cause your vehicle to catch fire.
The Signs of a Faulty Radiator Cap
As part of your monthly inspection, you should look closely at the radiator cap and the top of the radiator.
Leaks or Stains on the Radiator and Radiator Cap
If you see signs of coolant or antifreeze, either a pool of fluid or greenish, white, or reddish stains, your radiator cap is likely to fail. You should naturally also check that the radiator cap is twisted fully closed.
Your radiator pressure needs to be maintained at between 13 and 16 pounds per square inch (PSI), depending on the manufacturer’s specifications. This pressure is the result of the coolant heating up and expanding.
The radiator cap’s job is to keep the pressure constant, either allowing a little pressure to bleed off or allowing fluid to flow back into the radiator when the pressure falls below the minimum required pressure.
A leaking radiator cap indicates that the fluid is not able to pass into the expansion reservoir or that there is damage to one or both seals inside the cap.
Expansion Reservoir Overflowing
When the pressure exceeds the specified level for your cooling system, the valve in the radiator cap opens to allow fluid to travel into the expansion tank.
Once the pressure is relieved, the valve closes and the fluid remains inside the reservoir.
When the pressure drops below the minimum level, the radiator cap valve opens, allowing fluid to get sucked back into the radiator until the pressure rises to the required PSI.
If the expansion reservoir overflows, it means that after the radiator cap opens, it is not closing again when the pressure is relieved. The fluid then continues to flow into the reservoir until it overflows.
This may not be the only reason the reservoir overflows, as the cooling system may not be functioning correctly due to an electric fan not switching on or a blockage in the radiator or one of the coolant hoses.
This will cause the temperature and the pressure to rise in concert, forcing the coolant into the expansion reservoir as the pressure keeps rising.
Air Bubbles in the Cooling System
One of the jobs of the radiator cap is to prevent air from entering the cooling system. It does this by sealing the spout with the top seal.
If the seal is faulty, air will be drawn into the radiator as the radiator fluid cools down.
The air forms bubbles in the coolant, interfering with the flow of coolant through the radiator core as well as the water pump.
The interrupted flow of coolant will cause the engine to overheat.
The easiest place to see bubbles will be in the expansion reservoir.
However, there are other reasons why air could enter the cooling system, such as a blown head gasket, failing water pump seals, heater seals, and faulty overflow reservoir.
Radiator Hose Bursts
A sure sign that there are problems with your cooling system and radiator cap is when you find coolant sprayed around the inside of the engine bay.
As the cooling system heats up, coolant will be forced out of any crack, tear, or pinprick hole. What makes this difficult to find is that often the system will seal up when the engine cools down.
However, the radiator cap is the likely culprit. By failing to open properly when the pressure rises, the radiator cap prevents pressure bleeding from the cooling system resulting in leaks and ultimately a burst coolant hose.
Radiator Hose Collapses
The opposite of a burst hose is when a radiator hose collapses.
The valve in the radiator cap will allow the increased pressure to bleed off into the expansion reservoir, but when it comes time for the valve to allow the coolant to return to the radiator, it fails to open.
The increasing vacuum created by the contraction of the coolant volume inside the radiator will cause the radiator hoses to buckle under the pressure, eventually collapsing in on themselves.
This can damage the hoses, causing them to crack and later leak.
When inspecting and adding coolant, do so when the engine is cool or cold. Also, don’t try to inspect the radiator by taking the radiator cap off when the engine is running, as this will allow air to enter the cooling system.
If you want to add coolant, wait for the engine to cool down. Adding coolant to a hot radiator may result in you overfilling the radiator, which can cause your expansion reservoir to overflow or burst when the engine is hot.
FAQ – Frequently Asked Questions About What Problems Can A Loose Radiator Cap Cause
How do you test a radiator cap?
Before removing the radiator cap to test it, allow the engine to cool down. Check the inside top seal and the lower seal for nicks or tears. Then push on the bottom of the cap to see that the spring compresses which will allow coolant to flow to the expansion tank. Then use a cooling system & radiator cap pressure tester to see that the cap can hold its pressure.
Should I see the coolant when I open the radiator cap?
You should see the coolant when you open the radiator cap. The coolant level should be close to or at the level of the radiator cap spout. If the coolant level is much lower or if you can’t see any coolant at all, then you should add antifreeze up to the level of the radiator spout.
Why is my radiator full but the reservoir empty?
When your radiator is full it means that it has the correct amount of fluid. As the fluid heats up while your engine is running, it expands and pushes against the seal in your radiator cap. When the pressure rises above 13-16 pounds per square inch, the radiator cap seal opens and allows fluid into the expansion bottle. When the pressure reduces, the fluid flows out of the reservoir and back into the radiator.
Conclusion On What problems can a loose radiator cap cause
A loose or damaged radiator cap can cause your engine to overheat and result in costly repairs, not to mention a great deal of inconvenience if your vehicle breaks down.
To avoid problems with your radiator cap and cooling system, regularly check your coolant levels at least once a month.
Topping up your radiator if necessary and checking for leaks will help to quickly identify a loose or failing radiator cap.